Presently, the desalting membrane of choice worldwide is the polyamide (PA) membrane. In general, PA membranes are made by forming a thin PA film on the finely porous surface of a polysulfone (PS) supporting membrane by an interfacial reaction between the reactant pair trimesoyl chloride (TMS) and m-phenylenediamine (MPD). The following equation illustrates the chemical formation of the PA desalination barrier.

In this equation, the first term represents m-phenylenediamine in water, the second term represents the trimesoyl chloride in hydrocarbon, and the resultant term represents the fully aromatic polyamide thin film. This is the equation for the PA thin-film composite membrane developed by Cadotte and others (see, e.g., J. E. Cadotte, J. J. Peterson, R. E. Larson and E. E. Erickson, “A new thin-film composite seawater reverse osmosis membrane,” Desalination, 32, 25-31 (1980)) and, as indicated above, is the membrane in common use throughout the world.
A great need exists to improve the stability of the present state-of-the-art membranes used for chlorine disinfection. Such improvement is critical for Reverse Osmosis (RO) plants operating on wastewaters, surface waters and open seawater intakes wherein disinfection by chlorination is required to control the growth of microorganisms (so-called “biofouling”) on the surface of the membrane. These PA membranes are so susceptible to deterioration by chlorine that dechlorination is required when chlorine is used as a disinfectant in the pretreatment. It will be understood that dechlorination prior to the PA membrane creates additional costs and effectively nullifies disinfection on the membrane surface where disinfection is needed. It is also noted that such dechlorination does not neutralize all chlorine, and the small amount of residual chlorine shortens membrane life.
It will be appreciated that there is a serious need for a chlorine-resistant PA membrane since such membrane would have significantly increased life, would prevent biofouling and lower the overall cost of desalting. In spite of claims by some manufacturers that their membranes last longer than competitive membranes in low concentrations of chlorine, it has been found that all of these PA membranes degrade and lack chemical stability to oxidants such as chlorine. However, as indicated above, chlorine is very effective biocide in water treatment and thus its use is quite desirable. If a truly effective chlorine resistant membrane could be provided, desalting plants and mobile desalting units could operate in a more robust manner, while decreasing costs of membrane cleaning, storage and replacement and of general overall operations.